For the past 20 years, people have been analyzing the difference between tubes and transistors, this has been particularly true in the audio world. The primary difference between the two has to do with how they operate in the nonlinear region. Tubes tend to distort in an asymmetrical manner (relative to the Q or operating point), and transistors tend to distort in a more symmetrical manner with respect to the Q point. Herein lies the big difference. If a sine wave is input into a tube amplifier, which is operating in the nonlinear region, the output will not exhibit half wave symmetry due to the asymmetrical distortion. The bottom line of this is that even ordered harmonics will be promoted. If a sine wave is input into a transistor amplifier, operating in the nonlinear region, half wave symmetry will prevail in the output signal, due to the symmetrical distortion properties of the transistor. The bottom line of this is that odd ordered harmonics will be promoted. The even ordered harmonic generation is apparently more pleasant to the human ear, thus resulting in a big resurgence in popularity for tube amplifiers.
However, tubes can act like transistors depending on the circuit they are used in. A perfect example of this is in a classic push/pull circuit. One tube in the circuit distorts the top half of the signal, and the other tube in the circuit (operating 180 degrees out of phase) distorts the lower half of the signal in the same manner. Hence, half wave symmetry is promoted and even ordered harmonics are cancelled. Basically, symmetrical push/pull circuits cancel even ordered harmonics and promote odd ordered harmonics when driven into the nonlinear range.
The system of the present invention (a single input push/pull circuit), does exactly the opposite. It cancels odd ordered harmonics and promotes even ordered harmonics. Furthermore, it has become very desireable to match the tube pairs used in push/pull amplifiers, by performance criteria. The system of the present invention eliminates the need to match the tubes due to the feedback path in the circuit. However, high quality, high performance tubes are still recommended.
It should be known that this circuit can also be applied to transistors, or any other solid-state device that emulates the transfer characteristics of a vacuum tube. FETs (field effect transistors) act in this manner if biased correctly.
Many inventions of past address push/pull circuits but don't address the cancellation of even ordered harmonics, the matching of the active components, or the ability to drive the circuit with one signal which affords the designer the ability to apply filtering in the final (push/pull) stage without dual-ganged components.
U.S. Pat. No. 4,021,684 doesn't employ feedback in the same manner as the present invention, and creates symmetrical distortion. U.S. Pat. No. 4,987,381 is nothing more than a FET push/pull amplifier that keeps the preamp in the linear region while saturating the push/pull stage. U.S. Pat. No. 5,032,796 provides asymmetrical distortion but does not address this feature in a push/pull configuration. U.S. Pat. No. 4,710,727 generates distortion but doesn't address a push/pull configuration. Nor does it allow for feedback to compensate for the difference between components if a push/pull amplifier were used for item #305 in the diagram. U.S. Pat. No. 4,079,332 requires two inputs 180 degrees out of phase and doesn't incorporate feedback. U.S. Pat. No. 4,811,401 again is not a push/pull amplifier. U.S. Pat. No. 5,127,059 is a preamp, not a push/pull amplifier. U.S. Pat. No. 4,415,865 is a control for the quiescent current of an AB push/pull amplifier and doesn't address asymmetrical distortion. U.S. Pat. No. 4,644,289 is very similar to the present invention in that it can generate asymmetrical distortion, however, this distortion is generated in a preamp stage and it doesn't address feedback for the purpose of compensating for the difference in component performance.